(1) Field of the Invention
The present invention relates to a disk apparatus of a sector-servo type in which head positioning information is recorded on a data plane of a disk medium and, more particularly, to a disk apparatus capable of recording and reproducing variable-length data, as well as a recording/reproduction method therefor.
(2) Description of the Related Art
Disk apparatuses, such as magnetic disk apparatuses and optical disk apparatuses, are widely used as external storage devices for computer systems.
In a recent disk apparatus, a bit pitch in the circumferential direction of the disk medium and a track interval (or a track pitch) in the radial direction of the same have been made smaller in order to increase the recording density of the disk medium in an effort to satisfy a demand for an increased storage capacity.
In addition to a servo-plane-servo type disk apparatus and a sector-servo type (i.e., data-plane-servo type) disk apparatus, there exists a hybrid type disk apparatus.
In the servo-plane-servo type disk apparatus, servo information for use in positioning a head is recorded on a servo plane of a disk medium. A dedicated servo head is used to detect the servo information. A data head facing a data plane is subjected to position control on the basis of the thus-detected servo information. The data head records data on or reproduces data from the disk medium.
In the sector-servo type disk apparatus, the servo information is recorded on the data plane of the disk medium. This servo information is directly detected by use of the data head, and the positioning of the data head is controlled on the basis of the detected servo information. The data head records data on or reproduces data from the disk medium.
The hybrid type disk apparatus is a hybrid of a servo-plane-servo type disk apparatus and a sector-servo type disk apparatus. In the disk apparatus of this type, the servo information is recorded on both the servo plane and the data plane of the disk medium in a split manner. A dedicated servo head is used to detect the servo information recorded over the entire memory area on the servo plane. The positioning of the data head is roughly controlled on the basis of the thus-detected servo information. Subsequently, the servo information for on-track control purposes recorded on the data plane is directly detected by the data head. On-track control of the data head is carried out on the basis of the servo information detected from the data plane, whereby the data head records data on or reproduces data from the disk medium.
However, a thermal off-track problem sometimes arises in the servo-plane-servo type disk apparatus. More specifically, the data head fails to follow the track of the data plane even when the servo head properly follows the track of the servo plane. In such a case, the data head is positioned off the track center and, therefore, is unable to correctly record data on or reproduce data from the disk medium. For these reasons, it has been difficult to improve the recording density of the disk medium by reducing the track pitch.
In contrast, in the sector-servo type and hybrid type disk apparatuses, the data head can precisely position itself at the track center by virtue of the servo information recorded on the data plane. Even if the track pitch is reduced, it is possible for the data head to correctly record data on or reproduce data from the disk medium. In consequence, the recording density of the disk medium can be improved.
The disk medium is formatted in one of two ways; namely, a sector format and a count key data format (CKD format).
In the sector format, a record section comprises an address section (ID section) and a fixed-length data section. In contrast, in the CKD format, the record section comprises a count section, a variable-length key section (this key section is optional), and a variable-length data section.
In the sector format comprising the fixed-length data section, the servo information is recorded at predetermined intervals to be located at the boundary of each record section (i.e., a sector). For this reason, it is possible to identify the position of the recorded servo information. Therefore, the sector-servo type and hybrid type disk apparatuses can carry out the recording/reproduction of data having a fixed-length form while avoiding the servo area on the data plane where the servo information is recorded.
In the case of the CKD format comprising the previously described variable-length data section and key section, the position in each record section of the track on the data plane where the servo information is to be recorded is indefinite. Therefore, it is difficult to identify the position at which the servo information is recorded. In such a case, when variable-length data are recorded or reproduced, it is impossible to avoid the servo area where the servo information is recorded. In other words, the CKD format cannot be applied to the sector-servo type and hybrid type disk apparatuses.
In consideration of the length of the data actually produced by the sector-servo type and hybrid type disk apparatuses, it is desirable for the sector-servo type and hybrid type disk apparatuses to employ the CKD format that deals with variable-length data, as this would save the storage capacity of the disk apparatus.
For this reason, there has already been in actual use a disk apparatus that records or reproduces data by converting the sector format, which is a fixed-length data format, into the CKD format, which is a variable-length data format.
Even in this type of disk apparatus, variable-length data are recorded across a plurality of sectors after having been divided into data having a fixed length. Depending on the length of variable-length data, an unused area may frequently remain in the last sector at which the data are recorded. Because of this, the storage capacity of the disk apparatus cannot be effectively used.
In the sector-servo type and hybrid type disk apparatuses having a plurality of disk mediums, when data are recorded on or reproduced from the track of another disk medium after data have been recorded on or reproduced from the track of one disk medium, heads are switched (i.e., a head switching operation is carried out) so that the data can be recorded/reproduced using the head that corresponds to each disk medium.
In some of the previously described disk apparatus, the head is correctly positioned at the track center at the time of the head switching operation, through correction of the thermal off-track, which has been previously measured. Subsequently, a home address (HA), which designates a track address, is read.
However, the starting position of the track of each disk medium is provided at the same location in such a disk apparatus. Therefore, if the amount of correction of the off-track of the head becomes large after the head switching operation, it becomes impossible to complete the correction of the off-track of the head before the reading of the home address (HA) is started. As a result, it may sometimes be impossible to correctly read the home address (HA).
In this case, the home address (HA) will be read again after the disk medium has fully rotated once. Accordingly, it takes much time to record/reproduce the data after the head switching operation, and this considerably impairs the performance of the disk apparatus.